I realised that cheaper availability of night soil might be the reason behind the farmers motivation. I don't know if class actions are an option there, but if many fall sick because of their lack of responsibility, maybe making them compensation via court could set an example. At the end of the day it is farmers who have to realise that their practices are not good so govt intervention may help, and lack of intervention can make govt compliant for making people sick. I think a third option could be targeting the buyers. If a few vegetables providers start a venture with value proposition of providing parasite free veggies and become successful it can force the others to join the race. Which makes me wonder do people (vegetable consumers) care about getting these worm infections?